The phase separation process of the spinodal decomposition induced by the temperature of the polymer blend(PMMA/PVAc) possessing a upper critical solution temperature was measured by a light scattering method. Effects of the temperature and component of polymer on the periodic distance and the process of phase separation were studied. Initial stage of the phase separation can be interpreted by the linear theory of spinodal decomposition. Late stage of the phase separation can be correlated by the scaling theory.In the casting process the phase separation induced by the concentration change of the polymer was measured by the light scattering method. The bicontinuous structure in the casting film was formed by the spinodal decomposition The periodic distance was dependent on the evaporation rate and polymer quantity. The periodic distance can be quantitatively correlated by the lapse of the phase separation process. The process can be simulated by the diffusion equation described the phase separation.The polymer blend film possessing the separation function was prepared by the casting method. The periodic distance can be controlled by the evaporation rate. The separation of ethanol and water with this blend film was studied by the pervaporation method. The flux and separation factor can be correlated by the structure of the blend film.